It is known in the art to make passive strain sensors employing an optical fiber having intracore Bragg gratings as reflectors embedded therein. A Bragg grating, as is known, reflects only a narrow wavelength of light incident thereon, i.e., it has a narrow wavelength reflection band. Bragg gratings can be holographically written into the core of many experimental and commercially available fibers, as described in commonly owned U.S. Pat. No. 4,725,110 to Glenn et al, entitled "Method For Impressing Gratings Within Fiber Optics", and U.S. Pat. No. 4,807,950 to Glenn et al, entitled "Optical Fiber With Impressed Reflection Gratings".
Such passive sensors typically involve injecting a broadband source light into one end of a fiber having a plurality of Bragg gratings written successively therein, each having a different central reflective wavelength. A portion of the source light is reflected off a first grating having a reflective wavelength associated with the first grating. The rest of the source light passes through the first grating and on to a second grating in the fiber where again a portion is reflected having a wavelength associated with the second grating, and so on. The reflected light is detected by a wavelength detector at the end of the fiber where the source light was injected.
If one of the gratings in the fiber is subjected to a perturbation, e.g., a mechanical strain, a pressure, or a temperature change, the perturbation alters the optical path length between successive elements within the Bragg grating, thereby shifting the reflection wavelength of the Bragg grating. This wavelength shift may then be detected by the wavelength detector. The amount of wavelength shift is indicative of the size of the perturbation imposed on the fiber at the location of the grating. Thus, optical fiber Bragg grating sensors can be attached to or embedded in a medium to detect a wide range of environmental perturbations. This passive sensor technique is described in U.S. Pat. No. 4,806,012 to Meltz et al, entitled "Distributed, Spatially Resolving Optical Fiber Strain Gauge", and U.S. Pat. No. 4,761,073 to Meltz et al, entitled "Distributed, Spatially Resolving Optical Fiber Strain Gauge".
When a low power broadband light source is used, the resulting signal-to-noise ratio is quite low because only a small spectral portion of the light is reflected. For example, for a broadband source emitting 100 .mu.Watts at 50 nm, and a 50% grating reflector having a 0.1 nm wavelength bandwidth, the return power of the light is about 0.1 .mu.Watts, provided there are no other losses in the system (i.e., 100% system efficiency). Also, other system losses further contribute to signal degradation, such as fiber coupling losses or fiber mismatches, or other fiber splice losses. This results in a low signal-to-noise ratio of the detected signal. A low signal-to-noise ratio requires long filter time constants in the detector thereby resulting in reduced sensor response time, and/or reduces the accuracy of the measurement in noisy environments.
Alternatively, if a narrow band light source is used, all the power is typically concentrated into a narrow bandwidth which is smaller than the span of fiber grating wavelength shift over the sensed perturbation operating profile. Thus, narrow band lasers must be frequency tuned or scanned to track the grating reflection wavelength shift over the range of operating conditions.
Therefore, it is desirable to provide a sensor that does not have the aforementioned signal-to-noise problems associated with passive low power broadband source Bragg grating sensors, nor the frequency tuning/scanning requirements or other drawbacks of passive narrow band source Bragg grating sensors.